A gas insulated switchgear is generally composed of a plurality of switchgear units. To connect a main bus of a switchgear unit with that of its neighboring unit, a known way is to place a bus bar connection device that connects them (see Patent Reference 1). The bus bar connection device has insulation bushings at its both ends. Each insulation bushing has its inside shape like a cup and has an end of the main bus conductor in it. Each insulation bushing is placed on the housing side wall of the switchgear unit, with its nose passing through the wall into the unit. Each insulation bushing is placed so that one inside cup is opposite the other. To make a conjugate connection of conductor terminals inside both bushing cups, an insulation adapter with an axial conductor in it is inserted into both bushing cups.